In the case of a gas and steam turbine plant, the heat obtained from the operating means or the heating gas from the gas turbine is used to generate steam for the steam turbine. Heat is transferred to a waste-heat steam generator connected downstream of one of the gas turbines in which a plurality of heating panels are usually arranged to preheat the water in order to generate and superheat the steam. The heating panels are connected to the water-steam cycle of the steam turbine. The water-steam cycle usually includes a number of pressure stages, for example three, in which case, each pressure stage can feature an evaporator heating panel.
For the steam generator connected downstream on the heating gas side of the gas turbine as a waste-heat steam generator, several alternative embodiment concepts can be taken into consideration, namely, the embodiment as a continuous steam generator or the embodiment as a circulation steam generator. In the case of a continuous steam generator, when the steam generator pipes provided as evaporator pipes are heated, the flow medium in the steam generator pipes evaporates in a single through-flow. However, by contrast with this, in the case of a natural or forced circulation steam generator, the circulating water is only partially evaporated when flowing through the evaporator pipes. The water not evaporated in this case is again supplied to the same evaporator pipes for further evaporation after the generated steam has been separated.
Unlike a natural or forced circulation steam generator, the continuous steam generator is not subjected to a pressure limit so that in the case of initial steam pressures it can be embodied to exceed the critical pressure of water by far (PKri≈221 bar), in which case, it is not possible to differentiate between the water phase and the steam phase and, as a result, a phase separation is also not possible. A high initial steam pressure favors a high thermal degree of effectiveness and, therefore, low CO2 emissions of a fossil-heated power plant. In addition, a continuous steam generator compared to a circulation steam generator has a simple embodiment and can, as a result, be manufactured particularly cost-effectively. Therefore, the application of a steam generator embodied according to the through-flow principle as a waste-heat steam generator of a gas and steam turbine plant is, in this case, particularly favorable for obtaining a high overall degree of effectiveness of the gas and steam turbine plant with a simple embodiment.
Particular advantages with respect to manufacturing costs, but also with respect to maintenance work are offered by the horizontal waste-heat steam generator, for which the heating medium or the heating gas, that is the waste gas from the gas turbine is cross-flown in a more or less horizontal direction of flow through the steam generator. However, in the case of a horizontal steam generator, the steam generator pipes of a heating panel of the evaporator can, depending on their positioning, be exposed to greatly varying heating temperatures. Particularly in the case of the steam generator pipes of a continuous steam generator connected on the outlet side to a common accumulator, a different heating of the individual steam generator pipes, in each case, can lead to a joining together of the steam flows with the steam parameters deviating strongly from one another and, as a result, to undesirable losses in the efficiency, particularly to a relatively drop in efficiency of the heating panel involved and resulting reduced steam generation. A difference in heating of neighboring steam generator pipes can, in addition, damage the steam generator pipes or the accumulator particularly in the joining area of the accumulators in each case. Thus the desirable application of a horizontal continuous steam generator, in itself, embodied as a waste-heat steam generator for a gas turbine may cause considerable problems with respect to a sufficiently stabilized flow control.
From EP 0 944 801 B1, a steam generator designed for horizontal use is known and it also has the above-mentioned advantages of a continuous steam generator. In addition to this, the heating panel of the evaporator of the known steam generator is arranged as a continuous heating panel and is embodied in such a way that one pipe of the steam generator which is hotter than the other pipe of the steam generator of the same continuous heating panel of the evaporator has a flow medium rate which is higher than that of the other pipe of the steam generator. Thus, the continuous heating panel generally means a heating panel which is embodied for a cross-flow according to the through-flow principle. The flow medium supplied to the heating panel of the evaporator arranged as the continuous heating panel, therefore, completely evaporates in a single through-flow in each case through this continuous heating panel or through a heating panel system comprising a plurality of continuous heating panels which are connected in series to each other.
The evaporator panel of the evaporator of the known steam generator arranged as a continuous heating panel therefore shows, in the nature of the flow characteristics of a heating panel of a natural circulation evaporator (natural circulation characteristics) in the case of a different heating of the individual pipes of a steam generator, a self-stabilizing behavior, which without the requirement of external influences adjusts the temperatures on the outlet side even in the case of differently heated pipes of the steam generator which are connected in parallel on the flow medium side.
For this design of steam generator, in order to obtain a particularly low load through thermally-related stresses particularly in relation to the manufacturing and assembly costs kept particularly low in relation to the distribution of the flow medium on the water side and/or the steam side, the continuous heating panel of the evaporator of the steam generator can be designed as U-shape comprising a plurality of pipes of a steam generator which are connected in parallel to each other for through-flow of the flow medium, which each feature an almost vertically arranged down pipe section through which the flow medium can flow in a downwards direction and connected downstream from this on the flow medium side an almost vertically arranged riser pipe through which the flow medium can flow in an upwards direction. As has been shown, with this type of design, a pressure contribution through the geodetical pressure of the water column in the down pipe of the specific pipe of the steam generator can be utilized in a way that favors and promotes flow when the continuous heating panel is cross-flown.
However, such a design could basically promote the occurrence of flow instabilities on operating the continuous heating panel of the evaporator which could bring about operational disadvantages. Although supplying the pipes of the steam generator forming the continuous heating panel with a relatively low mass flow rate density and the relatively low frictional pressure loss associated with these allows the natural circulation characteristics of the flow in the pipe of the steam generator to be obtained, which has a stabilizing effect on the flow. Nevertheless, it is also desirable especially in the case of such a design with a pipe section which can be cross-flown downwards to contribute, to a particular extent to stabilizing the flow ratios when the continuous heating panel of an evaporator is operated.